Carburized components typically exhibit high hardness and low to moderate toughness. It is well known that one method of increasing the contact fatigue and scoring resistance of a conventionally carburized component is to utilize processes which form hard particles such as carbides in the surface microstructure, such as the process disclosed in U.S. Pat. No. 4,921,025 by Tipton et al. and assigned to the assignee of the present invention.
It is also well known that one method to increase toughness is to carburize and then austemper to produce a tougher case microstructure consisting primarily of lower bainite. This process of carburizing and austempering is commonly referred to as "Carbo-Austempering", and is known to increase the toughness of carburized components because at an equivalent hardness, a bainitic microstructure is tougher than a conventional martensitic microstructure. However, the accompanying hardness reduction results in undesirable lowering of wear, contact fatigue, and scoring resistance.
Carbo-Austempering of low and medium carbon steels is described in an article by W. R. Keough, titled Carbo-Austempering, published in 1995 (Carburizing and Nitriding with Atmospheres, Proceedings of the Second International Conference on Carburizing and Nitriding with Atmospheres, December, 1995, ASM International). However, it is commonly known by those skilled in the art that austempering of low and medium carbon steels, such as SAE8615, SAE4122, and SAE4150, effectively increases toughness, but result in lower surface hardness.
Steel articles produced by the process described in the above-referenced U.S. Pat. No. 4,921,025 have a plurality of carbides formed on the surface which provide high surface hardness. However, the articles, even though formed of lower to medium carbon steel, have relatively low toughness properties because the carbides are distributed in primarily a martensitic case microstructure.
It is therefore desirable to have a steel article, and a method of forming the article, that has both high surface hardness and higher toughness, without having a high core carbon content or the addition of relatively expensive carbide forming elements.